The present invention relates to a variable coupling resistance delay line for a crossed field tube.
Crossed field tubes are essentially constituted by a vacuum enclosure containing two parallel electrodes between which there is a potential difference creating a continuous electrical field E.sub.o. A magnetic field B is established in a direction perpendicular to the electrical field and to the tube axis, when the tube is linear and in a direction parallel to the tube axis and consequently perpendicular to the electrical field when the tube is cylindrical.
The positive electrode is constituted by a delay line, which generally has a periodic structure having a sequence of fingers facing the negative electrode and along which moves the microwave.
The negative electrode or sole can be non-emissive. In this case the tube incorporates an electron gun located at one end of the line and which produces an electron beam collected by a collector located at the other end of the line. This type of tube is called an injected beam tube.
The negative electrode can also be constituted by a cathode of the magnetron type. The cathode emission started by the high frequency electrical field extisting at the high frequency input of the tube is then amplified and maintained by the secondary emission phenomenon. This type of tube is called a distributed emission tube.
In the case of cylindrical distributed emission tubes the electron beam produced by the cathode generally returns on itself after crossing a degrouping space separating the input from the output of the high frequency wave. Such a tube is called a re-entrant beam tube.
A distinction is made between forward or backward wave crossed field tubes depending on whether the microwave travels in the direction of the electron beam in the delay line or in the reverse direction. The electron beam is always constituted by an electron layer located around the negative electrode and a number of space charge arms equal to the number of wavelengths delayed by the line. The space charge arms travel at a velocity V.sub.e equal to the phasse velocity V.sub..phi. of the microwave on the delay line. Most of the electrons of the arms fall on the line to which they transfer their potential energy, which ensures an amplification of the microwave signal.
Most crossed field tubes function as amplifiers, but there are also crossed field oscillators such as the carcinotron.
The carpitron is a carcinotron synchronized by a pilot means and is used as an amplifier. Carpitrons and carcinotrons are reverse wave, injected beam tubes.
The present invention relates to all types of crossed field tubes.
It is known from the prior art that the efficiency of crossed field tubes increases with the coupling resistance R.sub.c of the tube. The coupling resistance is written: EQU R.sub.c =(E.sup.2.sub.HFx)/(2.beta..sup.2 .multidot.P.sub.HF),
with
E.sub.HF x, the amplitude of the microwave field level with the line and parallel thereto; PA1 .beta., the propagation constant equal to 2.pi./.lambda..sub.r, in which .lambda..sub.r represents the delayed wavelength; PA1 and P.sub.HF the microwave power at any point x of the line.
It is the microwave field which acts on the electrons. Consequently the coupling resistance determines the action on the beam of the microwave field of the line. The coupling resistance varies in the opposite sense to the capacitance between two successive fingers of the line. Thus, if the interdigital capacitance increases, the microwave energy stored between the fingers of the line increases and the action on the beam of the microwave field of the line and consequently the coupling resistance decrease. Thus, the dimensions of the line fingers must be reduced for reducing the interdigital capacitance and increasing the coupling resistance.
The problem which arises is that the thermal conductivity is reduced on reducing the dimensions of the fingers, which can be dangerous due to the electron bombardment.
French Pat. No. 1 150 045 relates to a crossed field tube with an injected beam and backward wave functioning as an oscillator, i.e. a carcinotron. In this carcinotron action takes place on the height of the fingers which is their dimension in a direction perpendicular to the electrodes in order to make the coupling resistance low level with the electron gun where a large amount of heat has to be dissipated due to the electron bombardment on the line. The coupling resistance is then increased in a linear manner by moving away from the electron gun.